Wj. Parker et al., Pilot plant study to assess the fate of two volatile methyl siloxane compounds during municipal wastewater treatment, ENV TOX CH, 18(2), 1999, pp. 172-181
The environmental fate of octamethylcyclotetrasiloxane (D-4) and decamethyl
cyclopentasiloxane (D-5) during municipal wastewater treatment has been inv
estigated in an activated sludge pilot plant by continuously dosing the inf
luent with exagerated amounts of the compounds and measuring their emission
s in the off-gas, effluent, and sludge streams. The total removal was found
to be 86.4 +/- 3.9% for D-4 and 95.8 +/- 1.5% for D-5 (+/-1 SD statistical
uncertainty). However, low overall mass balances were found for both compo
unds in the pilot system. In order to elucidate the detailed removal mechan
isms for these two compounds, the model TOXCHEM was used to simulate their
removal and analyze their mass balances in the pilot system. Experimental a
nd model results indicated that the low mass balances were caused by the si
gnificant underestimation of the primary sludge removal by the grab samplin
g method used for sludge analyses. This underestimation maybe due to the hi
gh variability of the primary influent suspended solids concentration and h
ence inhomogeneity in the primary sludge underflow. When this underestimati
on was taken into account, primary sludge removal and volatilization in the
aeration basin were found to contribute approximately equally to the remov
al of D-4 and D-5 in the pilot plant system. The present results are useful
for the environmental risk assessment of these two siloxane compounds in t
he unlikely event of their point source discharge into municipal wastewater
systems.